I'm not sure of offhand stacks Dmg on ww. My off hand axe adds like 12k dps to stats. Let me know if u find a food answer
offhand weapon dps doesnt affect ww, but does affect tornados. the dps boost ur getting is most lkely from str / crit on ur offhand
also attack speed affects the proc rate on tornados, so more attack speed you have, the more times your tornados will proc ( have chance to crit, leech life blah blah blah)